Developing efficient, stable, and cost-effective photosystems to split water into hydrogen and oxygen using sunlight is of paramount importance for future production of fuels and chemicals from renewable sources. However, the high cost of current systems limits their widespread application. Here, we developed a highly efficient TiC-supported Pt nanocluster catalyst for hydrogen evolution reaction that rivals the commercial Pt/C catalyst with 5 times less Pt loading. Combining with the NiFe-layered double hydroxide for oxygen evolution reaction and driven for the first time by a monolithic perovskite/silicon tandem solar cell, we achieved a solar water splitting system with 18.7% solar-to-hydrogen conversion efficiency, setting a record for water splitting systems with earth-abundant and inexpensive photo-absorbers.